In commonly assigned U.S. Pat. No. 4,688,311 issued Aug. 25, 1987 and U.S. Pat. No. 4,998,580 issued Mar. 12, 1991, the details of which are herein incorporated by reference, there are disclosed heat exchangers and methods of making the same which employ flattened tubes which, in turn, have a plurality of internal, hydraulically parallel flow paths of relatively small hydraulic diameter, i.e. a hydraulic diameter of about 0.07 inches or less. Hydraulic diameter is as conventionally defined, namely, the cross-sectional area of the flow path multiplied by four (4) and divided by the wetted perimeter of the flow path.
Exceptional improvements in heat transfer are achieved utilizing such tubes, particularly in air conditioning applications where heat is being transferred between the ambient and a refrigerant flowing through the tubes.
Moreover, the use of tubes having flow paths of relatively small diameter allows the manufacture of a heat exchanger with a reduced internal volume. When the heat exchanger is used in a refrigeration system, this feature minimizes the refrigerant charge required and thereby minimizes the potential amount of an environmentally hazardous refrigerant (e.g. chloroflourocarbons) that may leak to the environment in the event of a leak in the system.
Further, the efficiency of heat exchangers using such tubes is such that a heat exchanger having a heat exchange capacity equal to that of a prior art heat exchanger can be made and have only a fraction of the weight of the prior art heat exchanger. This is a particular advantage in automotive air conditioning systems because the weight reduction will ultimately show up as an improvement in fuel efficiency.
It is believed that the relatively small hydraulic diameters of the flow paths in such tubes advantageously take advantage of surface tension and capillary effects to achieve improvements in heat transfer as more fully explained in the above identified '580 patent.
In addition, where the tubes are fabricated by the method disclosed in the previously identified '311 patent, the interior refrigerant flow passages within the tubes will be provided with so-called microcracks as a consequence of residual brazing flux remaining from the NOCOLOK brazing process. This is also as more fully explained in the previously identified '580 patent and is believed to provide additional heat transfer efficiencies as well.
Still further, surface irregularities in the flow passages of tubes formed by extrusion methods are also believed to act just as the microcracks or surface irregularities caused by the flux residue to provide the same efficiencies in heat transfer.
The present invention seeks to provide a new and improved heat exchanger that makes use of one or more of the foregoing advantageous characteristics.